Measurement of photon–jet transverse momentum correlations in 5.02 TeV Pb + Pb and pp collisions with ATLAS

Jets created in association with a photon can be used as a calibrated probe to study energy loss in the medium created in nuclear collisions. Measurements of the transverse momentum balance between isolated photons and inclusive jets are presented using integrated luminosities of 0.49 nb⁻¹ of Pb + Pb collision data at s_NN=5.02 TeV and 25 pb⁻¹ of pp collision data at s=5.02 TeV recorded with the ATLAS detector at the LHC. Photons with transverse momentum 63.1<p_T ^γ<200 GeV and |η^γ|<2.37 are paired with all jets in the event that have p_T ^jet>31.6 GeV and pseudorapidity |η^jet|<2.8. The transverse momentum balance given by the jet-to-photon p_T ratio, x_Jγ, is measured for pairs with azimuthal opening angle Δϕ>7π/8. Distributions of the per-photon jet yield as a function of x_Jγ, (1/N_γ)(dN/dx_Jγ), are corrected for detector effects via a two-dimensional unfolding procedure and reported at the particle level. In pp collisions, the distributions are well described by Monte Carlo event generators. In Pb + Pb collisions, the x_Jγ distribution is modified from that observed in pp collisions with increasing centrality, consistent with the picture of parton energy loss in the hot nuclear medium. The data are compared with a suite of energy-loss models and calculations.